SAN FRANCISCO – A person with knowledge of the injury says San Francisco 49ers nose tackle Glenn Dorsey has torn his left biceps muscle and will undergo surgery.

It's unclear how long he might be sidelined. Dorsey left Friday's practice with the injury. The source said Saturday that Dorsey had torn his biceps and would need an operation, speaking on condition of anonymity because the team had not announced anything about Dorsey's condition.

More tests and details are expected post-surgery.

The Niners had a day off from training camp Saturday. Fellow defensive lineman Ray McDonald played last season with a biceps tear.

Losing Dorsey would be a huge blow 11 months after 2013 starter Ian Williams went down with a season-ending ankle injury in a Week 2 loss at Seattle.